This invention relates to the retreading of tires. The invention relates in particular to a method of and apparatus for the retreading of tires, and to tires retreaded in accordance with the method of the invention.
According to the invention there is provided a method of retreading tires, the method including the steps of positioning a tread over the tread zone of a tire casing with a bonding material interposed between the tread and the tire casing, enclosing the assembly of tread and tire casing inside a flexible envelope with the envelope covering the external surface of the tyre casing and tread assembly as well as the internal surface of the tire casing, positioning the assembly of tire casing and envelope on a rim, forcing the envelope against the tread and the tire casing, and stressing the tire casing by fluid pressure in the annular cavity defined between the rim and the assembly of tire casing and envelope so that the tread zone is in a stressed condition when the tread is being bonded thereto.
The fluid pressure in the annular cavity may be provided by a gas under pressure.
A fluid may be introduced at ambient temperature under pressure into the annular cavity and the temperature of the fluid may then be raised to thereby cause an increase in the fluid pressure in the cavity.
The envelope may be forced against the tread and the tire casing by sucking out air from the space between the envelope and tread and tire casing. Alternatively, or in addition, fluid pressure higher than atmospheric pressure is applied to the external surface of the envelope.
Conveniently, the air may be sucked out from the envelope prior to fluid pressurisation of the annular cavity.
The method may include providing an expandible tube inside the cavity and providing fluid pressure inside the tube to thereby pressurize the cavity and thus to expand the tire casing. The expandible tube may be fast with the flexible envelope and in at least a part of the flexible envelope covering the internal surface of the tire casing may form a part of the expandible tube.
In an alternative arrangement the stressing of the tire casing by fluid pressure in the annular cavity may be effected without employing a tube in the annular cavity.
The invention also extends to tires whenever retreaded by the method in accordance with the invention.
Further according to the invention there is provided tire retreading apparatus including a flexible envelope which can totally enclose the assembly of a tire casing and a tread positioned on the tread zone of the tire casing so that the envelope covers the internal and the external surfaces of the assembly, a tubular rim on which a tire casing enclosed in the envelope is locatable so that the tire is axially restrained on the rim and so that an annular cavity is defined between the rim and the tire casing enclosed in the envelope located on the rim, the annular cavity being capable of being pressurized by fluid pressure to force the envelope against the internal surface of the tire casing to thereby stress the tire casing.
The rim may be a composite structure comprising two tubular parts which are axially releasably attachable to one another. One end of each rim part be flared out and may form a shoulder engageable with the bead region of a tire casing for axial restraint of the tire casing on the rim.
Conveniently, the rim parts may have releasably lockably interengageable formations. The formations on one rim part may comprise hooks and the formations on the other rim part may comprise projecting studs which are engageable by the hooks by relative rotational movement of the rim parts with respect to one another. Conveniently, the formations are provided on the interior tubular surfaces of the rim parts.
The releasably lockably interengageable formations may maintain the rim parts with an axial spacing between them when the formations are locked together.
There may be provided a seal which is adapted to seal the axial joint between the rim parts in a fluid tight manner when they are in an operative position. Conveniently, the seal is an annulus of a resilient material and has a cross-sectional profile of H-shape which is engageable with the rim parts. The seal and the rim parts may have complementary interengageable formations to improve the sealing of the surfaces of contact between the rim parts and the seal.
In one embodiment of the apparatus, the annular cavity may be capable of being pressurized by fluid pressure without employing a tube in the annular cavity, there being provided a valve in the rim to introduce fluid pressure through the rim into the annular cavity.
In an alternative embodiment, the apparatus may include an expandible tube which is positionable in the annular cavity and is expandible by fluid pressure to thereby pressurise the cavity and stress the tire casing. The tube may be an annulus of an elastic material which is expandible by being inflatable by a gas under pressure. In a particular arrangement, the expandible tube may be fast with the flexible envelope and at least a part of the flexible envelope covering the internal surface of the tire casing may form a part of the expandible tube.
The flexible envelope may be strengthened in the region where it contacts the marginal axial regions of the rim. The strengthening may be by means of a strip of material which is adhesively secured to the envelope. If desired, the strip of material may be reinforced.
The expandible tube may have a tubular projection through which the tube can be inflated, and the rim may have an aperture through which the tubular projection can project.